Manual delayed s3 registration

NAMESPACE

@@ -171,11 +171,3 @@ importFrom(tweenr,tween_numeric)
 importFrom(tweenr,tween_state)
 importFrom(utils,modifyList)
 importFrom(utils,packageVersion)
-
-if (getRversion() >= "3.6.0") {
-  S3method(knitr::knit_print, 'gganim')
-  S3method(knitr::knit_print, 'gif_image')
-  S3method(knitr::knit_print, 'video_file')
-  S3method(knitr::knit_print, 'sprite_image')
-}
-

NEWS.md

@@ -1,5 +1,7 @@
 # gganimate (development version)
 
+* Fix registration of S3 methods for suggested packages
+
 # gganimate 1.0.1
 
 * Fix bug in transitions when the group aesthetic was late evalued (#262)

R/zzz.R

@@ -0,0 +1,32 @@
+.onLoad <- function(...) {
+  register_s3_method("knitr", "knit_print", "gganim")
+  register_s3_method("knitr", "knit_print", "gif_image")
+  register_s3_method("knitr", "knit_print", "video_file")
+  register_s3_method("knitr", "knit_print", "sprite_image")
+
+  invisible()
+}
+
+register_s3_method <- function(pkg, generic, class, fun = NULL) {
+  stopifnot(is.character(pkg), length(pkg) == 1)
+  stopifnot(is.character(generic), length(generic) == 1)
+  stopifnot(is.character(class), length(class) == 1)
+
+  if (is.null(fun)) {
+    fun <- get(paste0(generic, ".", class), envir = parent.frame())
+  } else {
+    stopifnot(is.function(fun))
+  }
+
+  if (pkg %in% loadedNamespaces()) {
+    registerS3method(generic, class, fun, envir = asNamespace(pkg))
+  }
+
+  # Always register hook in case package is later unloaded & reloaded
+  setHook(
+    packageEvent(pkg, "onLoad"),
+    function(...) {
+      registerS3method(generic, class, fun, envir = asNamespace(pkg))
+    }
+  )
+}